1. Field of the Invention
This invention relates to the apparatus for thermoplastic tubing having annular ribs, the apparatus being of the type in which tubing of the thermoplastic material is continuously extruded into a tubular mold tunnel.
2. Background of the Invention
The tubular mold tunnel is defined by cooperatively interengaged and mold blocks of a pair of mold assemblies, the mold blocks of each mold assembly being driven along a forward run in which the blocks define the modular block tunnel, and back along a return run. The mold blocks have molding surfaces comprising troughs for molding ribs of the tubing, and crests for molding grooves of the tubing. Such an apparatus is disclosed for example in U.S. Patent No. 3,981,663 issued on Sept. 21, 1976 to Gerd P. H. Lupke, and in 4,712,993 issued on Dec. 15, 1987 to Manfred A. A. Lupke.
With such known apparatus, the annular extrusion orifice is defined by a nozzle and conical mandrel disposed coaxially within the nozzle and connected to a downstream cylinder or sleeve which functions to guide the molten thermoplastic material toward the mold cavity and to provide the smooth inner surface of the pipe. In the apparatus described in the U.S. Pat. Specification No. 3,998,579, the sleeve is disposed a considerable distance downstream from the nozzle with the result that the molding space defined by the nozzle, the mandrel and the sleeve has an excessive volume. In the use of such known apparatus, the thermoplastic material is cooled excessively and becomes too stiff to flow into the rib recesses in the mold blocks. This problem cannot easily be avoided by raising the temperature of the material since that presents a serious risk that the material might burn. Polyvinylchloride (PVC) is a very useful thermoplastic material for forming pipes and this material will disintegrate if the temperature is raised too high.
In an attempt to avoid this problem, it has been proposed to reduce the axial extend of the extrusion orifice to such an extent that it is less than the distance between two adjacent annular recesses in the mold blocks, i.e. the extrusion orifice has an axial length less than the space between two annular ribs on the product pipe. While this procedure avoids cooling and flow problems associated with the use of large molding volumes, another problem arises due to the variation in the requirement of plastic material during operation of the apparatus. This variation results from the different thicknesses of pipe wall required for ribs and for the grooves. Thus, a considerable amount of thermoplastic material is required to fill the annular troughs in the mold blocks to form the ribs of the tubing. Less material is required at the crests of the mold blocks for the thinner walled grooves of the tubing. This calls for a very critically controlled flow of plastic material which is difficult to achieve. Even when the flow is satisfactory, pulsed stresses are set up at the Point of filling the mold blocks due to the varying demand for plastic material.